1. Field of the Invention
The present invention concerns an apparatus for making a plurality of cellular phones randomly access a common channel to transmit data in CDMA (Code Division Multiple Access), and a method therefor.
2. Technical Background
Recently, it has been demanded that the mobile communication system may provide data communication services to enable exchanges of moving image and character data together with those of conventional voiced information during movement IMT-2000 (International Mobile Telecommunication-2000) is an international project to develop a mobile communication system to enable such data communication services based on an internationally integrated standard. In addition, an effort has been made to achieve high speed data transmission with some limitation in the mobile characteristics, and particularly, the wireless LAN proposed in IEEE 802.11 standard is commercially able to make data transmission at 2 Mbps (Mega bit per sec).
Generally, the circuit-switch, which is employed in the present public switched telephone network (PSTN), digital cellular system and personal communication system, is not desirable to make such high speed data transmission because of ineffectively using a limited number of channels. In order to resolve the disadvantages of the circuit-switch, the packet-switch has been proposed to make high speed data transmission in IMT-2000, which is under development for LAN or the standard of the future mobile communication system. The band spread method of CDMA applied to the present digital cellular system has a large communication capacity, and is prevented from external tapping and stabilized to make communications under the multipath environment generated by the radio channels rapidly changed through movement, so that it is widely used in the commercial wireless LAN, CDMA One of LMNQ (Lucent Motorola, Nortel, Qualcomm, Samsung) proposed as the standard of the future IMT-2000, or W-CDMA (Wideband-CDMA) chiefly developed in Europe and Japan.
The future IMT-2000 system is to support both voiced data and packet data communications. In this case, it is very ineffective that the packet data is assigned with an exclusive channel as the voiced data. Namely, although the continuous voiced data must be assigned with an exclusive channel, it is desirable for effective use of resources and simplification of the system that the discontinuous small packet data are transmitted through the reverse common channels less than the number of the active terminals. Moreover, if the number of the subscribers is increased, and thus the amount of the data transmitted, the reverse common channels must be necessarily used for the effective use of resources.
The ALOHA method is the most effective way of channel access and data transmission through the reverse common channel, and a typical random access, which is developed by Hawaii University in 1970 as a protocol for the wireless network between the islands of Hawaii State. However, such ALOHA suffers the inefficiency of data transmission performed without any timed schedule between the base station and terminals, and network overloaded due to frequent collision of data transmitted by a number of terminals. The slotted ALOHA has been developed to define a standard time between the base station and terminal in order to cope with such problems. This method is to allow the data transmission of a terminal only in a set time, so that the rate of substantial data and thus the transmission efficiency is increased, and the interferences from another terminal is reduced, thereby improving the efficiency of the whole network. However, since the slotted ALOHA makes the data transmission basically in the contention mode, it is impossible to avoid collisions of data transmitted from the terminals. Hence, if there is an additional channel for controlling and transmitting information in the forward link from the base station to the terminals, the base station broadcasts the information of the received data to the terminals in order to avoid such data collision.
In the ALOHA system employing CDMA, each terminal uses a different code or the same code with a time offset to avoid data collision. In addition, a preamble signal is firstly transmitted to determine the power to be used for data transmission, and the initial synchronization with the code to use and the tracking of the code synchronization is performed previous to the data transmission, thereby preventing data transmission error. Describing the conventional ALOHA protocol employing CDMA in reference to FIG. 1, a terminal makes an access attempt to transmit data. The access attempt consists of a plurality of access sub-attempts, each of which in turn consists of a plurality of access probe sequences, each of which in turn consists of a plurality of access probes. The access probe consists of a preamble transmitting a pilot channel without any substantial information to evaluate the channel situation and to make the initial synchronization between the codes of the terminal and base station and the tracking of the code synchronization, and an access channel message capsule containing the access information or the user's data following the preamble. In order to transmit an access signal or data through the reverse common channel, a terminal firstly transmits an access probe at a given power level, which must have a minimum value to reduce the interferences affecting the other terminals that transmit data through an exclusive channel or other reverse common channels. Then, the terminal which made an attempt to transmit data with the first access probe waits for a prescribed time interval TA the signal from the base station representing whether the transmitted data is received. If the first access probe fails to reach the base station, the terminal makes another attempt to retransmit the second probe at a power level increased by a given level PI compared to the power level of the first probe after a random time interval RT. If not succeeding in the data transmission through a prescribed number of the access probes of the first access probe sequence, the terminal performs the second access probe sequence after passing an additional random time interval RS.
While the conventional ALOHA protocol system employing CDMA can make a stable data transmission in the reverse common channel by performing stably the code synchronization and the tracking thereof with the preamble of a sufficient length even when many data are transmitted through the reverse common channel, the preamble is the signal to establish the code synchronization, and less important and longer than the substantial data. Thus, if the synchronization is exactly acquired in the preamble but with an error in transmission of the substantial data, the preamble should be retransmitted reducing the data transmission efficiency.